This invention pertains to electrochemical cells and more particularly is concerned with treating cells having lithium metal anodes with oxyhalide-type electrolytes.
Such cells are capable of being stored for extended time periods without significant degradation of their capacity. One reason for the long shelf life is that the surface of the lithium metal anode starts to oxidize almost immediately upon contact with the oxyhalide electrolyte. A dense oxidizing film forms on the anode surface which protects the anode from further chemical corrosion or oxidization. This phenomenon is known as passivation. When the cell is placed under load after a period of storage there is a delay before the cell voltage raises to an acceptable level. Apparently, the film which protects the anode during storage prevents a sufficient number of lithium ions from going into solution until a discharge current flows for a time. The delay time varies from seconds to hours, depending on the cell construction and storage conditions with high rate cell particularly affected. The delay in reaching operating voltage is a problem for those applications where almost instantaneous electrical power is required of a cell.
A very efficient type of a primary cell has a lithium metal anode and an electrolyte solution of thionyl chloride (LiSOCl.sub.2) and lithium tetrachloroaluminate (LiAlCl.sub.4). It is known to partially discharge such cells before storage as the current flow causes SO.sub.2 to be generated from the reduction of thionyl chloride. The SO.sub.2 reduces start-up delay time. The predischarge, however, depletes the capacity of the cell.
It is an object of the invention to treat lithium metal anode cells to reduce start-up delay time without depletion of cell capacity.